Oscillator



p 1953 E. J. H. BUSSARD ET AL 2,854,573

OSCILLATOR Original Filed Oct. 18. 1951 2 Sheets-Sheet 1 m. mm W? m w WW! N w wubhn uflw A m m my EB P 1958 E. J. H. BUSSARD ET AL 2,854,578

OSCILLATOR Qpiginal Filed Oct. 18. 1951 2 Sheets-Sheet 2 huuaz Lma 58EqmvAuauT O F Q01 22 l Lane AND F as I 69 2 72 1 W1 7/ IIVVVENTORS. IW.- EMMERY .1. H. BUSSARQ 68 BY REUBEN NATHAN I10 2 66 ATTORN Y5.

United States Patent F OSCILLATOR Emmery J. H. Bussard and ReubenNathan, Cincinnati, Ohio, assignors to Avco Manufacturing Corporation,Cincinnati, Ohio, a corporation of Delaware Original applicationsOctober 18, 1951, Serial No. 251,864, now Patent No. 2,763,776, datedSeptember 18, 1956, and October 29, 1952, Serial No. 319,622. Dividedand this application December 15, 1953, Serial No. 406,034

3 Claims. (Cl. 25036) The present invention relates toultrahigh-frequency (U. H. F.) converters for television receivers. A U.H. F. converter is a device which selects the radio frequency carriersignals in the desired U. H. F. channel, converts them into firstintermediate frequency (I. F.) carrier signals in thevery-high-frequency (V. H. F.) range, and then applies the first I. F.output signals to the V. H. F. signal input circuit of a televisionreceiver tuner. A V. H. F. tuner is a unit included in the receiver,comprising preselector circuits, a local oscillator and a mixerfunctioning cooperatively to select carrier frequency signals in thedesired V. H. F. channel, to convert them into intermediate frequencysignals (referred to as second I. F. signals when a converter is used)and to apply these I. F. signals to the conventional intermeidatefrequency amplifier stages of the receiver. When a U. H. F. converter isused in conjunction with a V. H. F. tuner the selector circuits of theV. H. F. tuner are adjusted to receive the V. H. F. signal output of theconverter, and the receiver and converter function together as a doublesuperheterodyne receiver.

Subject matter disclosed but not claimed herein is disclosed and claimedin United States Patent 2,763,776, of which the instant application is adivision, and in another divisional application which was filed October29, 1952, and bears Serial No. 319,622, of which the instant applicationis also a division.

In the illustrative U. H. F. converter herein shown, the frequency ofthe local oscillator is lower than the frequency of the U. H. F. signalinput to the converter, this tuner being intended for use with areceiver having a non-symmetrical intermediate frequency system and alocal oscillator operating at higher frequencies than that of the V. H.F. input to the receiver proper. Provision is made in this manner forcorrect presentation of signals to the intermediate frequency systemincluded in the receiver. In the alternative, when a converter isemployed with a receiver in which the local oscillator frequency islower than the frequencies of the V. H. F. input to the receiver, thenthe frequency of the local oscillator included in the convertor shouldbe made higher than that of the U. H. F. signal input to the converter.

At the present time channels Nos. 2 through 13 are available in theUnited States for commercial video broad- 'casting, with V. H. F.channel frequency allocations as 2,854,578 Patented Sept. 30, 1958 ICCThe complete V. H. F. range comprises a lower V. H. F. band (54-88megacycles) and an upper V. H. F. band (174-216) megacycles). In thepreferred embodiment of the present invention, this factor is exploitedto great advantage, the first I. F. output signal frequencies of theconverter being in the portion of the spectrum between those two bands.This portion is not used at any place in the United States for videobroadcasting.

The present invention generically embraces, but is not specificallylimited to, a converter having a V. H. F. signal output frequency withinone of the present V. H. F. channels. A converter which is so limited isdesigned for a very wide bandwidth to provide output I. F. frequenciescovering two adjacent V. H. F. channels, so that an alternate channelmay be used for U. H. F. reception if the other V. H. F. channel isassigned to the location where the converter is installed. Prior artconverters which provide a V. H. F. signal output frequency within thepresent V. H. F. channels are subject to a further limitation, even whendesigned to provide output frequencies covering two adjacent V. H. F.channels, because they do not operate in a satisfactory manner in areaswherein both channels are used for V. H. F. reception. Prior art tunersof this character may be tuned to provide output frequencies withineither of two present V. H. F. channels. The present invention affords avery significant advantage in that a V. H. F. selector used inconjunction with our novel converter may be adjusted to receive I. F.signals at any point within the receiver pass band, and such selector isnot limited to two positions.

The preferred embodiment of the present invention has a narrowerbandwidth and is advantageously used with a continuous type of V. H. F.tuner, the output I. F. frequencies being in the portion of the spectrumbetween the V. H. F. bands, the portion being covered by continuous V.H. F. tuners but not by step-by-step tuners. It is, accordingly, anobject of the preferred form of the invention to provide:

First, a converter having a narrower output bandwidth;

Second, a converter which can universally be used with continuoustuners;

Third, a converter which provides output carrier signals in the portionof the spectrum between the V. H. F. bands;

Fourth, a converter which does not require a range of output frequenciescovering two adjacent V. H. F. frequencies; and

Fifth, a converter having enhanced gain, signal-to-noise ratio andselectivity characteristics.

The Federal Communications Commission presently contemplates theallocation of carrier frequencies from 470 to 890 megacycles totelevision broadcast transmission and proposes to add to the present V.H. F. channels a total of 70 additional channels, Nos. 14 through 83,comprising the U. H. F. band or range. Upon the completion and finaladoption of this allocation plan or a similar proposal, commerciallysuccessful television re-. ceivers will require:

A combined U. H. F.-V. H. F. tuner for the selection of any one of thevery large number of channels within the U. H. F. and VJH. F. ranges, or

A U. H. F. converter in combination with a V. H. F.

receiver.

V U. H. F. converters will then be required in large numbers to adapt V.H. F. receivers to U. H. F. reception; The preferred type of converterin accordance with the invention will have V. H. F. output frequenciesbetween the V. H. F. bands. Other converters, including a modified formin accordance with the invention, will'have V. H. F. output frequenciesin one of the present V. H. F.

channels.

Other important objects of the invention are to provide:

First, a converter which features novel double-tuned bandpass selectorand local oscillator circuits;

Second, a converter which minimizes oscillator radiation;

Third, a converter including an oscillator having a novel andparticularly stable bridge type feedback system;

Fourth, a converter having uniform mixer excitation from the localoscillator.

For a better understanding of the invention, together with other andfurther objects, advantages, and capabilities thereof, reference is madeto the following description of the accompanying drawings, in whichthere is shown an oscillator in accordance with the invention:

Fig. 1 is an electrical schematic of the circuits included in theconverter;

Fig. 2 is a circuit diagram of the novel oscillator included in theconverter; and

Fig. 3 is an equivalent circuit diagram used as an aid in explaining theoperation of the Fig. 2 circuit.

The novel converter unit in accordance with the invention comprises thefollowing major units, all as shown in Fig. l and in U. S. Patent2,763,776, to which reference is made for a description of the entireunit: First, a doubie-tuned bandpass preselector circuit comprising thetuning lines 20 and 21 and immediately associated components; second, acrystal mixer diode 22 to which the selected radio frequency carriersignals are applied; third, a local oscillator comprising vacuum tube23, tuning line 24 and associated components for generating localoscillations which are also applied to the crystal mixer to convert, byheterodyne action, the carrier frequency signals into intermediatefrequency signals; fourth, a low noise stage of first I. F. poweramplification comprising a vacuum tube 25 and associated circuitelements; fifth, a power supply in the form of a half-wave rectifierinclusive of tube 26, functioning as a source of heater and spacecurrents; and sixth, a ganged pair of control switches 27 and 28,manually operable to condition the receiver for ultra-high-frequencyoperation (U. H. F.) or very-high-frequency operation (V. F. H.).

A suitable UHF antenna is connected to antenna input terminals 29 and 36mounted on insulating board 31. These terminals are connected byconductors 32 and 33 to the primary of an antenna input transformer,which primary comprises a loop of conductive material 34, one terminalof which is grounded at 35. The first preselector circuit comprises aparallel-conductor type of tuning line 20 which is adjusted by ashortcircuiting bar, indicated by the reference numeral 36, to produceparallel resonant conditions in the tuned circuit comprising tuning line20, end inductor 37, trimmer capacitor 38, capacitor 39, and metallicplate 40. Plate 40 is a ribbon conductor which serves both as aninductor and as the fixed plate of a capacitor, in furtherance of thetwo functions of antenna coupling and coupling between the two circuitsof the selector network. The closed end of transmission line 20 isgrounded at 41, and the adjustable shorting bar is grounded at 4.2. Oneterminal of line 20 is connected to plate 40, and the other terminal isconnected at point 43 to an adjustable end inductor 37. The remainingterminals of plate 40 and inductor 37 are connected, respectively, tothe high potential terminals of capacitor 39 and capacitor 38. Capacitor38 is adjustable and is connected to ground at 44. The remainingterminal of capacitor 39 is grounded at 45. The antenna input primary 34is coupled to the first preselector circuit, inclusive of theelements'39, 40, 20, 37, and 38, by the capacitive and mutuallyinductive relationship existing between loop 34 and plate 40.

The bandpass selector network includes a second tuned preselectorcircuit comprising tuning line 21 and associated circuit elements 46,47, 48, 49, and 50. Line 21 is provided with an adjustable shorting bar51; The

closed end of the tuning line is grounded at 52. One terminal of thetuning line is connected to a terminal of capacitor 46. The otherterminal of tuning line 21 is connected at 53 to adjustable end inductor47. Capacitor 48 is connected between grounded point 54 and theremaining terminal of inductor 47. Capacitor 46 projects through thechassis and is connected to crystal 22 at junction 55. Inductance 50 isconnected between point 55 and ground, and capacitor 49 is alsoconnected between point 55 and ground. Point 55 is the junction ofcrystal 22, capacitors 46 and 49, and inductance 50.

The first preselector circuit is coupled to the second preselectorcircuit by the capacitive and inductive, primarily the capacitive,relationships between plate 40 and plate 19, plate 19 being connected tothe junction of capacitor 46 and tuning line 21.

The preselector output is taken from terminals 55 and 54 (ground), andthe parallel combination of capacitor 43 and inductor 50 is connectedacross these terminals.

The preselector circuitry between the antenna input terminals and themixer 22 having been described in detail, the discussion now proceeds tothe oscillator circuit shown in Fig. 1. The oscillator comprises a tube23 which is placed in a shielding can. The oscillator tube is mounted ona socket. The two oscillator grid terminals are connected to oneterminal of an adjustable end inductor 58, the remaining terminal ofwhich is connected to a terminal of capacitor 59. The other terminal ofcapacitor 59 is connected to tuning line 24 at point 60, and the line isconnected at this point to a plate 61 which is mounted in spacedrelationship to another plate 62 to form an adjustable capacitor, plate62 being connected to both oscillator tube anode terminals. Disposedimmediately above and adjacent capacitor 59 is a capacitor 63 whichconnects both anode terminals of tube 23 to the remaining terminal ofthe tuning line. A grid resistor 64 is connected between the gridterminal of tube 23 and grounded point 65. One of the heater terminalsis grounded, and the other heater terminal is connected at 66 to thejunction of a resistor 67 and an inductance 68. The cathode terminal isreturned to ground through an inductance 6?. The anode of oscillatortube 23 is connected to the space current supply source through seriallyrelated resistors 70 and 71, a filter capacitor 72 being connectedbetween the junction of these two resistors and ground. The injectioncircuit between the oscillator and the mixer originates at theoscillator heater and is completed through resistor 67 and capacitor 73,the latter being connected between crystal 22 and resistor 67. Point 81is the junction of crystal 22 and capacitor 73. A capacitor 17 isconnected between point 81 and ground 16.

Between the closed end of tuning line 24 and ground is connected aparallel combination of resistance 74 and capacitance 75. The oscillatortuning line is provided with a shorting bar 191.

The oscillator heater circuit connections to the heater current supplyare completed through the parallel combination of inductor 68 andresistor 76 to a terminal 77 (Fig. l). Inductance 68 is mounted onresistor 76. A shunt capacitor 78 is connected between junction 77 andground, conductor 79 thence leading to the filament current supplyterminal 97.

Coming now to a description of the method by which this converter isaligned, a metering circuit comprising a rectifier and a high-gainoscilloscope is inserted between points 55 and 81 (Fig. l), and thecrystal 22 is open-circuited. Converter power is turned off, and thetuning shaft is turned to the maximum clockwise or highest frequencyposition, the pointer on thetuning dial (not shown) then being setagainst a limit stop located slightly to the right of the channel 82calibration on the dial. Next the dial is set at 700 megacycles, andthere signal of 685 to 715 megacycles. Trimmer capacitors 38 and 48 arethen adjusted to maximum oscilloscope deflection, and capacitor 19, 40is adjusted until the oscilloscope pass band pattern flat tops. The dialis then set at 470 megacycles, and a 400 cycle amplitude modulatedsignal on a 470 megacycle carrier is applied to the antenna inputterminals 29, 30. Capacitors 38 and 48 are again adjusted to maximumoscilloscope deflection. Finally the dial is set at 890 megacycles, anda 400 cycle amplitude modulated signal on an 890 megacycle carrier isapplied to the U. H. F. antenna input terminals 29, 30, whereupon theend inductors 37 and 47 are adjusted for maximum oscilloscopedeflection. The foregoing steps are repeated if necessary. The dial isagain set to 700 megacycles and, using. a sweep signal of 685 to 715megacycles, the capacitor 19, 40 is again adjusted until theoscilloscope pass band fiat tops.

The oscillator is now adjusted, power is turned on, and the dial set atthe maximum clockwise position. Inductance 58 is then adjusted until theoscillator frequency is 775 megacycles, utilizing an insulated alignmenttool. Opening the end inductor 58 lowers the oscillator frequency, andclosing it increases that frequency. Finally the dial is set at themaximum counterclockwise position and capacitor 61, 62 is adjusted untilthe oscillator frequency is 338 megacycles. An oscillator frequencyrange from 338 to 775 megacycles is appropriate for a converter outputsignal frequency approximating 127 megacycles. V

The converter is connected to a Crosley continuous tuner, adjusted to127 megacycles, and the converter is turned on. By the use of atraveling detector and band pass indicator, the over-all passmband ofthe converter is peaked at 124 and 130 megacycles by adjustment of thecore of transformer 111, the core of transformer 84, and trimmercapacitor-156. The connections to the Crosley receivers having a Crosleycontinuous tuner are made with a 300 ohm twin transmission line. CrosleyV. H. F. tuners of the type suitable for use in conjunction with thisconverter, as indicated by the block marked 200 in Fig. 1, are shown inthe following patents of Emmery I. H. Bussard, assigned to the sameassignee as the present application and invention (to wit, AvcoManufacturing Corporation) U. S. Patent 2,652,487 Constant Band WidthCoupling Circuit for Television Receiver Tuners, and U. S. Patents2,615,983, 2,579,789, and 2,711,477, each entitled Tuner for TelevisionReceivers.

The oscillator generates local oscillations within the frequency rangefrom 338 to 775 megacycles, for a converter output frequency centered at127 megacycles. The oscillator circuit is illustrated in Fig. 2 and thebridge equivalent in Fig. 3. Connected between thesymmetricalanode'leads of triode 23 and the symmetrical grid leads ofthat tube are a series combination of a first capacitor 63, tuning line24, fixed capacitor 59, and adjustable end inductance 58, the lattercomprising a bendable strip of conductive material. The elements 63, 24,59, and 58 are equivalent to the two arms L C and L C of the bridgenetwork shown in Fig. 3. The tuning line is adjustably short-circuitedby a shorting bar 191, this element. 191 being only symbolicallyillustrated in Fig. 2. The shorting bar 191, like the other shortingbars, consists, of a contact mounted on the end of 'a suitably insulatedarm. The shorting bar is ganged for unicontrol with the preselectorshorting bars or contacts. The closed end of the line is connected toground through a parallel combination of resistor 74 and capacitor 75(Fig. 2), to providethe parameters R and C represented in Fig. 3. Theremaining arms of the bridge are provided by the grid-cathodeinterelectrode capacitance C and the plate-cathode interelectrodecapacitance C of tube 23, as represented in Fig. 3. A grid resistor 64is 6 connected between grid and ground, and the anode is connected tothe positive power supply line (-l-B) through a filter networkcomprising series resistors 70, shunt. capacitance 72, and seriesdropping resistor 71. In this manner the parameters R and R areeffectively provided.

In series between the cathode and ground is a choke 69 shown as L; inFig. 3. One terminal of the heater is grounded and the other heaterterminal is connected to the ungrounded filament current supply linethrough a filter comprising: first, a parallel combination including thechoke 68 and resistor 76, and second, a shunt capacitor 78., In parallelwith the cathode inductance L is the series combination of theheater-cathode capacitance C the heater resistance R and the heaterinductance L The circulating current in the oscillator tank circuit,consisting of the reactance arms of the bridge between grid and plate,produces out-of-phase potentials, required to sustain oscillations, atgrid and plate. The cathode is tapped in near a null point of the bridgeso that the reaction of the cathode and heater circuits on theoscillator tank circuit is minimized. The cathode inductance L andheater inductance L are resonated by the heater-cathode capacitance ofthe tube at approximately 7 0O megacycles. Mixer excitation is derivedby coupling through the heater-cathode capacitance, one terminal of theheater being placed in circuit with the crystal mixer 22 by a resistor67 and a capacitor 73 (Fig. 1).

The preselector circuit coupled to the mixer and the mixer itself have aminimum reaction on the oscillator tank circuit,'this desirable resultbeing obtained by taking the oscillator voltage from the heater circuit.The excitation voltage is taken across the parameters R and L which arerepresentative of the heater resistance, the heater self-inductance, andthe self-inductance of the choke 68.

To provide for factory adjustment, trimmer capacitor 61, 62is connectedbetween the anode of tube 23 and terminal of tuning line 24.

The oscillator injection is relatively uniform across the band. Thereaction of the heater inductance increases with frequency and tends toincrease the output as the transit-time loading of the input circuitincreases. This action compensates for the general tendency towardreduction in oscillator output voltage caused by the decrease ineffective R at higher frequencies.

As will be seen from an inspection of Fig. 3,tl1is oscillatoreffectively has plate and grid tank circuits. One of the parametersintercoupling thesetank circuits is the plate-gridinterelectrodecapacitance of tube 23, referred to as C Another is thevariable capacitor 61, 62. A third is the cathode choke 69 indicated asL in Fig. 3. This choke is, of course, a magnetic coupling parameter. Itfunctions to change the feedback ratio as operating frequency isincreased, to compensate for transit-time delay effects which increasewith increasing frequency. As these effects tend to attenuatethe localoscillation output, the feedback ratio is changed to increase the driveon the input of the oscillator tube and to maintain with reasonableconsistency the amplitude of the local oscillator output signals.

This oscillator circuit has excellent stability location of parts, bythe use of negative temperature coefficient capacitors 63, 75, 59, andby thermal isolation frequency made possible by symmetrical leads andconnections in the following manner: As clearly shown by the dispositionof the elements 58 and 63 in Fig. 2, we effectively couple a singletuning line 24 into the cert character- 1st1cs. In production weminimize oscillator drift by the tral points of symmetry of the plateand grid of tube 23, thereby realizing many of the advantages whichwould otherwise have to be achieved by the provision of two tuning linesin lieu of the single open-wire line 24 which this invention exploits.

The converter or frequency changing stage exploits a germanium crystalmixer 22. Carrier signal input to the mixer is provided by a connectionfrom junction point 55 (of capacitor 49 and inductor 50) to the cathodeof the crystal (Fig. 1). In most installations the polarity of thecrystal is immaterial. The combination 49, 50, considered alone, isdesirably resonant at approximately 310 megacycles. This combinationserves two useful purposes: (1) It attenuates oscillator voltagestending to radiate from the antenna, because it serves as an efiectiveshort circuit to such voltages, looking from the oscillator into theterminals 55, 54; (2) The signal coupling into the mixer provided by thepreselector and this parallel capacitor 49-inductor 50 combinationvaries automatically with preselector tuning in such a manner as tocompensate for the normal decrease in gain which accompanies an increasein signal frequency. The elements 4% and 50 are preferably designed, inconjunction with the preselector, for maximum power transfer of thecarrier signals to the mixer. These elements terminate tuning line 21 insuch a manner as to provide a proper coupling to a diode mixer. Localoscillation injection into the mixer stage is provided by capacitor 73and resistor 67, in series with the anode of crystal 22 and theoscillator tube heater, i. e., between junction points 65 and 81.Between point 65 and ground is a series circuit comprising: a parallelcombination of inductor 68 and resistor 76, and a capacitor 78.

The mixer and associated circuit elements accomplish in a novel mannerthe basic functions required of a frequency converter stage in asuperheterodyne receiver, to wit: First, the beating of the localoscillator frequency against the input carrier frequency to produce thedcsired difference frequency output; Second, the presentation of a lowinput impedance to intermediate frequencies; Third, the presentation ofa high input impedance at the mixer to R. F. carrier frequencies andlocal oscillations; Fourth, the rejection of sum frequencies and inputfrequency components in the mixer output system; Fifth, the rejection ofimage frequencies and undesired carrier frequencies preparatory toapplication of signals to the mixer.

At a given crystal excitation power, the crystal presents one impedanceto the carrier frequency circuit and another to the intermediatefrequency circuit. With conventional methods of oscillator coupling, theoscillator injection and hence the crystal excitation power would varyover Wide limits. We provide a novel oscillator injection circuit whichminimizes mismatch and improves mixer performance. Uniform oscillatorinjection not only minimizes mismatch, but it generally improves theefli ciency of mixer performance. One of the major advantages of thecrystal mixer is the possibility of supplying a lower excitation powerfor efficient mixer operation, dccrcasing oscillator radiation from theantenna.

As indicated above, the excitation voltage from oscillater to mixer istaken oif at the oscillator heater socket clip 66 so that the loadreflected into the oscillator tank circuit by the mixer and associatedcircuits is in bal anced relationship with respect to the feedbackbridge network in the oscillator. Thus the mixer and preselectorcircuits have a minimum reaction on the oscillator, and uniform mixerexcitation, oscillator range and oscillator stability are promoted. Itwill now be appreciated by those skilled in the art that a minimum ofoscillator tank circuit loading is achieved by driving the mixer from avoltage developed in the common leg of the feedback bridge network inthe oscillator.

While we do not desire to be limited to a single set of circuitparameters, the following illustrative parameters Resistor 149 820 ohms.

Resistor 153 820 ohms.

Resistor 157 27,000 ohms.

Resistor 103 10,000 ohms.

Resistor 64 10,000 ohms.

Resistor 70 1,800 ohms.

Resistor 9f. 220 ohms.

Resistor 57 180 ohms.

Resistor i0 1,300 ohms.

Resistor 71 5,600 ohms.

Resistor 76 330 ohms.

Resistor 74 1,000 ohms.

Tube 23 Type 6AF4.

Tube 26 Type 6X4.

Tube 25 Type 6BQ7.

Mixer 22 Type 1-N72.

Capacitor 39 1.5 micrornicrofarads.

Capacitor 38 .86.5 micromicrofarads,

variable.

Capacitor 19 .1-.5 micromicrofarad,

variable.

Capacitor 34,40 1.5 micromicrofarads.

Capacitor 46 2.2 micromicrofarads.

Capacitor :8 .8-6.5 micromicrofarads,

variable.

Capacitor 49 5 micrornicrofarads.

Capacitor 59" 6. micromicrofarads.

Capacitor 61 .1-1.5 micromicrofarads,

variable.

Capacitor 63 12 micromicrofarads.

Capacitor 17 1.0 micromicrofarad.

Capacitor 72 470 micromicrofarads.

Capacitor 73 2.2 micromicrofarads.

Capacitor 92 4.7 micromicrofarads.

Capacitor 78 470 micromicrofarads.

Capacitor 1500 micromicrofarads.

Capacitor 9? 1500 micromicrofarads.

Capacitor 116 1500 micromicrofarads.

Capacitor 155 micromicrofarads.

Capacitor 114 20 microfarads.

Capacitor 151 20 microfarads.

Capacitor 156 2 0400 microfarads,

variable.

Capacitor 154 16 microfarads.

Inductance 37 002-.0045 microhenry self-inductance, variable.

Inductance 47 .002-.0045 microhenry self-inductance, variable.

Inductance 53 .001-0025 microhenry self-inductance, variable.

Inductance 50 .05 microhenry self-inductance.

Inductance 69 .96 microhenry self-inductance.

Inductance 84 .162 to .238 microhenry self-inductance.

Inductance 94 .095 microhenry self-inductance.

Inductance 95 .095 microhenry self-inductance.

Inductance 68 .9 microhenry self-inductance.

Oscillator Tuner: 7

Distributed capacitance. 3.5 micromicrofarads,

maximum.

Maximum inductance .07445 microhenry.

Minimum inductance .033 microhenry.

9 Preselector Tuners:

Distributedcapacitance- Mixer 2.0 micromicrofarads. Antenna 1.7micromi'crofarads. Maximum inductance- Mixer .0654 microhenry. Antenna.0606 microhenry. Minimum inductance Mixer .0314 microhenry. Antenna0.323 microhenry. Oscillator range 338 to 775 megacycles. Converterrange 465 to 902 megacycles. First intermediate frequency 127.5megacycles. Over-all gain of converter 1.2-2.0. Voltages:

Plate, tube 23 100 volts. Plate of output section,

tube 25 225 volts. Cathode of input section, tube 25 2.0 volts.Resonance frequencies:

Elements 49, 50 310 megacycles. Primary of output transformer 123megacycles. Secondary of output transformer 131 megacycles. Inputimpedance of V. H. F.

receiver 150 ohms, approximately. Impedance of U. H. F.

antenna 150 ohms, approximately While there has been shown and describedwhat is at present considered to be the preferred embodiment of theinvention, it will be understood by those skilled in the art thatvarious modifications and changes and substitutions of equivalents maybe made therein within the true scope of the invention as defined by theappended claims.

Having fully disclosed and described our invention, we claim:

1. In a bridge-type oscillator for a converter, a vacuum tube having atleast cathode, control and anode electrodes and a separate heater, aheater choke connected to said heater, a frequency determining tankcomprising a short-circuited tuning line coupled between said anode andcontrol electrodes to constitute two arms of a bridge network, saidbridge network being completed by gridcathode and plate-cathodecapacitances, and means for providing a high impedance direct currentpath across said bridge, the last-mentioned means comprising a cathodechoke connected between said cathode and ground and a parallelresistance-capacitance combination connected between ground and theclosed end of said tuning 10 line, whereby such means ofiers a highimpedance to bridge unbalance, the heater circuit resistance andinductance parameters and the inherent heater-cathode capacitanceparalleling said cathode choke to provide a,

junction point for driving a mixer, said cathode and heater chokes beingresonated within the range of operating frequencies.

2. In a bridge-type oscillator for a converter, a vacuum tube having atleast cathode, control and anode electrodes and a separate heater, afrequency determining tank comprising a short-circuited tuning linecoupled between said anode and control electrodes to constitute two armsof a bridge network, said bridge network being completed by grid-cathodeand plate-cathode capacitances, and means for providing a direct currentpath across said bridge, the last-mentioned means comprising a cathodechoke connected between said cathode and a point of reference potentialand a parallel resistance-capacitance combination connected between saidpoint of reference potential and the closed end of said'tuning line,whereby such means offers a high impedance to bridge unbalance, theheater resistance and inductance parameters and the inherentheater-cathode capacitance paralleling said cathode choke to provide ajunction point for driving a mixer.

3. In a bridge-type oscillator for a converter, a vacuum tube having atleast cathode, control and anode electrodes and a separate heater, afrequency determining tank comprising a short-circuited tuning linecoupled between said anode and control electrodes to constitute two armsof a bridge network, said bridge network being completed by grid-cathodeand plate-cathode capacitances, and means for providing a direct currentpath across said bridge, the last-mentioned means comprising a cathodechoke connected between said cathode and a point of reference potentialand a parallel resistance-capacitance combination connected between saidpoint of reference potential and the closed end of said tuning line,whereby the leg comprising such means offers a high impedance to bridgeunbalance, and means for driving a mixer from said leg.

References Cited in the file of this patent UNITED STATES PATENTS2,104,916 Evans Jan. 11, 1938 2,405,229 Mueller et a1 Aug. 6, 19462,440,308 'Storck Apr. 27, 1948 2,551,228 Achenbach May 1, 19512,602,139 Hodder et al. July 1, 1952 2,618,748 Rust et a1 Nov. 18, 19522,740,889 Eckert Apr, 3, 1956 2,763,776 Bussard et a1 Sept. 18, 1956

